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Earths Galore

StarDate:

February 22, 2014

Two decades of planet hunting have yielded a bounty of more than a thousand known worlds in other star systems. In a galaxy that’s home to as many as 400 billion stars, though, a thousand planets isn’t even a drop in the bucket. Yet it is enough for scientists to start talking about the whole bucket — how many stars host planets, what kinds of planets, and much more.

Of particular interest are planets like our own — Earth-size worlds orbiting at the right distance from Sun-like stars. Temperatures on such worlds would be just right for liquid water — a critical ingredient for life as we know it.

Estimates of what fraction of Sun-like stars have such worlds have varied from almost nothing to as much as a third.

One recent estimate came from a study led by Geoff Marcy of UC-Berkeley. The study looked at observations of tens thousands of Sun-like stars made by the planet-hunting Kepler spacecraft. The study found about 10 planets that are roughly as big as Earth and orbit at the right distance for liquid water. The study team then made some adjustments to account for planets that Kepler might have missed and other factors.

The study concluded that if the rest of the galaxy is similar to our own stellar neighborhood, than almost a quarter of Sun-like stars should have Earth-size planets at the right distance for liquid water. In fact, the nearest such planet could be within a dozen light-years — a familiar-looking planetary neighbor.

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